GitLab

This approach is able to cope with `%` characters in the URL template,
which is important since `%` is a valid URL character. Additionally
this approach is less likely to fail on an invalid string. This is
important since the distributed tracing infrastructure is designed to
degrade gracefully when not properly configured, and a small mistake
in the configuration of the URL template could have led to a production
outage.

Jaeger is a distributed tracing tool. This change adds a "Tracing" link
to the performance bar to directly link to a current request in Jaeger.

This is useful for two reasons: 1 - it provides affordance to developers
that the distributed tracing tool is available, so that it can quickly
be discovered. 2 - it allows developers to quickly find a specific trace
without having to manually navigate to a second user-interface.

This change will instantiate an OpenTracing tracer and configure it
as the global tracer when the GITLAB_TRACING environment variable is
configured. GITLAB_TRACING takes a "connection string"-like value,
encapsulating the driver (eg jaeger, etc) and options for the driver.

Since each service, whether it's written in Ruby or Golang, uses the
same connection-string, it should be very easy to configure all
services in a cluster, or even a single development machine to be
setup to use tracing.

Note that this change does not include instrumentation or propagation
changes as this is a way of breaking a previous larger change into
components. The instrumentation and propagation changes will follow
in separate changes.